Cobalt oxide, composite of cobalt oxide, and method for making the same

ABSTRACT

A method for making a composite of cobalt oxide is disclosed. An aluminum nitrate solution is provided. Lithium cobalt oxide particles are introduced into the aluminum nitrate solution. The lithium cobalt oxide particles are mixed with the aluminum nitrate solution to form a mixture. A phosphate solution is added into the mixture to react with the aluminum nitrate solution and form an aluminum phosphate layer on surfaces of the lithium cobalt oxide particles. The lithium cobalt oxide particles with the aluminum phosphate layer formed on the surfaces thereof are heat treated to form a lithium cobalt oxide composite. The lithium cobalt oxide composite is electrochemical lithium-deintercalated at a voltage of Vx, wherein 4.5V&lt;Vx≦5V to form a cobalt oxide. The present disclosure also relates to a cobalt oxide and a composite of cobalt oxide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201110168996.3, filed on Jun. 22, 2011, in the China Intellectual Property Office, the contents of which are hereby incorporated by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to cobalt oxides, composites of cobalt oxides, and methods for making the same.

2. Description of Related Art

There are three stable oxide forms of cobalt, cobalt (II) oxide (CoO), cobalt (III) oxide (Co₂O₃), and cobalt (II,III) oxide (Co₃O₄). Although cobalt can have a valence of +4, a practical method for making a stable oxide of cobalt having +4 valence has not been achieved.

What is needed, therefore, is to provide a cobalt oxide, a composite of cobalt oxide, and a method for making the same to form a stable cobalt oxide having +4 valence of cobalt.

BRIEF DESCRIPTION OF THE DRAWING

Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments.

FIG. 1 is a schematic view of an embodiment of a method for making a composite of cobalt oxide.

FIG. 2 is a flowchart of an embodiment of the method for making the composite of cobalt oxide.

FIG. 3 is a schematic view of a structure of a layered type lithium cobalt oxide and an embodiment of cobalt oxide.

FIG. 4 shows a Scanning Electron Microscope (SEM) image of an embodiment of a lithium cobalt oxide composite.

FIG. 5 shows a Transmission Electron Microscope (TEM) image of the embodiment of the lithium cobalt oxide composite.

FIG. 6 is a graph of cycle performance testing results of half cells using the cathode electrode including the embodiment of the lithium cobalt oxide composite.

FIG. 7 is an X-ray diffraction (XRD) pattern of an embodiment of cobalt oxide.

FIG. 8 is a schematic view of a comparative example of a lithium cobalt oxide composite.

FIG. 9 shows a SEM image on low magnification of the example of the comparative lithium cobalt oxide composite.

FIG. 10 shows a SEM image on high magnification of the example of the comparative lithium cobalt oxide composite.

FIG. 11 is a graph showing cycle performance testing results of half cells using the example of the comparative composite of LiCoO₂.

DETAILED DESCRIPTION

The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “another,” “an,” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

Referring to FIG. 1 and FIG. 2, one embodiment of a method for making a cobalt oxide 120 including steps of:

S1, providing a lithium cobalt oxide composite 10; and

S2, obtaining cobalt oxide 120 by electrochemical lithium deintercalation in the lithium cobalt oxide composite at a voltage of Vx, wherein 4.5V<Vx≦5V, to deintercalate lithium ions from the lithium cobalt oxide composite 10.

The lithium cobalt oxide composite 10 has a core-shell structure, including lithium cobalt oxide particle 12 and an aluminum phosphate (AlPO₄) layer 14 coated on a surface of the lithium cobalt oxide particle 12.

A mass percentage of the AlPO₄ layer 14 to the lithium cobalt oxide composite 10 can be in a range from about 0.1% to about 3%. A thickness of the AlPO₄ layer 14 can be in a range from about 5 nanometers (nm) to about 20 nm. The AlPO₄ layer 14 can be an in situ formed layer on the surface of the lithium cobalt oxide particle 12 and is a continuous material layer of AlPO₄ having a uniform thickness. Furthermore, the interfacial diffusion may occur at the interface between the AlPO₄ layer 14 and the lithium cobalt oxide particle 12, and cobalt atoms of the lithium cobalt oxide particle 12 may diffuse into the AlPO₄ layer 14, to form a Co—Al—O co-oxide.

The material of lithium cobalt oxide particle 12 has a layered type structure, and can be doped or undoped. The layered type lithium cobalt oxide can be represented by a chemical formula of Li_(x)Co_(1-y)M_(y)O₂, wherein 0.1≦x≦1.1, 0≦y≦0.9. In one embodiment, 0.1<y<0.5. M represents at least one of the chemical elements of alkali metal elements, alkaline-earth metal elements, Group-13 elements, Group-14 elements, transition metal elements, and rare-earth elements. In one embodiment, M represents at least one of the chemical elements of manganese (Mn), nickel (Ni), chromium (Cr), cobalt (Co), vanadium (V), titanium (Ti), aluminum (Al), iron (Fe), gallium (Ga), neodymium (Nd), and magnesium (Mg). In one embodiment, the material of lithium cobalt oxide particle 12 is LixCoO₂.

It is noted that, in step S1, the provided lithium cobalt oxide composite 10 can include a plurality of lithium cobalt oxide particles 12 individually and entirely coated by the AlPO₄ layer 14. One particle of the lithium cobalt oxide composite 10 includes a single lithium cobalt oxide particle 12 and an AlPO₄ layer 14 coated on a surface of the single lithium cobalt oxide particle 12.

A diameter of the lithium cobalt oxide particle 12 can be in a range from about 100 nm to about 100 microns. In one embodiment, the diameter of the lithium cobalt oxide particle 12 can be in a range from about 1 micron to about 20 microns.

Lithium ions can be deintercalated from the lithium cobalt oxide particle 12 while the core-shell structure is maintained. Therefore, a composite of cobalt oxide 100 can be formed. The composite of cobalt oxide 100 has a similar core-shell structure as the lithium cobalt oxide composite 10. The composite of cobalt oxide 100 includes the cobalt oxide 120 and an AlPO₄ layer 14 coated on a surface of the cobalt oxide 120. The AlPO₄ layer 14 is individually coated on the entire surface of the particle of the cobalt oxide 120.

The cobalt oxide 120 has a layered structure. The material of the cobalt oxide 120 can be represented by a chemical formula of Co_(1-y)M_(y)O₂, wherein 0≦y≦0.9. In one embodiment, 0.1<y<0.5. represents at least one of the chemical elements of alkali metal elements, alkaline-earth metal elements, Group-13 elements, Group-14 elements, transition metal elements, and rare-earth elements. In one embodiment, represents at least one of the chemical elements of manganese (Mn), nickel (Ni), chromium (Cr), cobalt (Co), vanadium (V), titanium (Ti), aluminum (Al), iron (Fe), gallium (Ga), neodymium (Nd), and magnesium (Mg). More specifically, the cobalt oxide 120 can be cobalt (IV) oxide, wherein the valence of the cobalt is +4. The cobalt oxide 120 is a product of deeply lithium-deintercalated lithium cobalt oxide. In one embodiment, the material of cobalt oxide 120 is cobalt dioxide (CoO₂).

One embodiment of a method for preparing the lithium cobalt oxide composite 10 includes steps of:

-   -   S11, providing an aluminum nitrate (Al(NO₃)₃) solution;     -   S12, introducing a plurality of lithium cobalt oxide particles         12 into the Al(NO₃)₃ solution, and mixing the plurality of         lithium cobalt oxide particles 12 with the Al(NO₃)₃ solution to         form a mixture;     -   S13, adding a phosphate solution into the mixture to react with         the Al(NO₃)₃ solution and form an aluminum phosphate layer on         surfaces of the lithium cobalt oxide particles 12; and     -   S14, heat treating the lithium cobalt oxide particles 12 with         the aluminum phosphate layer formed on the surfaces thereof.

The Al(NO₃)₃ solution includes liquid phase solvent and Al(NO₃)₃ dissolved in the solvent. The solvent can dissolve the Al(NO₃)₃, dissociating the Al(NO₃)₃ into Al³⁺ and NO₃ ⁻. Therefore, the solvent is not limited to water and can be a volatile organic solvent. In one embodiment, the solvent can be at least one of ethanol, acetone, chloroform, diethyl ether, dichloromethane, and ethylidene chloride. A reaction may occur between the lithium cobalt oxide particles 12 and the water thereby deteriorating the performance of the lithium cobalt oxide particles 12. The organic solvent may avoid this unwanted reaction.

In the step S12, the lithium cobalt oxide particles 12 cannot be dissolved in the Al(NO₃)₃ solution. A mixing of the lithium cobalt oxide particles 12 and the Al(NO₃)₃ solution forms a mixture containing both liquid phase and solid phase. In the mixture, a layer of Al³⁺ is uniformly coated on the surface of the lithium cobalt oxide particle 12. The Al³⁺ ions can be uniformly adhered to the surface of the lithium cobalt oxide particle 12 to form a coating layer at the atomic level. Further, the amount of the lithium cobalt oxide particles 12 introduced to the Al(NO₃)₃ solution can be controlled according to the amount of the Al³⁺ ions in the Al(NO₃)₃ solution, such that the amount of the Al³⁺ ions is just sufficient to coat all the surfaces of the lithium cobalt oxide particles 12. The mixture can be pasty. The pasty mixture is formed when the amount of the Al(NO₃)₃ solution is just enough to coat the entire surfaces of the lithium cobalt oxide particles 12. The pasty mixture can be formed by controlling a volumetric ratio of the Al(NO₃)₃ solution and the lithium cobalt oxide particles 12. In one embodiment, the volumetric ratio of the Al(NO₃)₃ solution and the lithium cobalt oxide particles 12 can be in a range from about 1:10 to about 1:40. A diameter of the lithium cobalt oxide particles 12 can be smaller than 20 microns. The amount of the Al(NO₃)₃ in the mixture can be determined from the amount of the aluminum phosphate layer to be formed on the surface of the lithium cobalt oxide particles 12. In one embodiment, a mass percentage of the aluminum phosphate layer in the lithium cobalt oxide composite 10 can be in a range from about 0.1% to about 3%.

In the step S13, the phosphate solution includes a liquid phase solvent (e.g., water), and a phosphate that can dissolve in the solvent. The phosphate can be monoammonium phosphate (NH₄H₂PO₄), diammonium phosphate ((NH₄)₂HPO₄), ammonium phosphate ((NH₄)₃PO₄), phosphoric acid (H₃PO₄), or combinations thereof. The phosphate solution can include phosphate radical ions such as phosphate ion (PO₄ ³⁻), dihydrogen phosphate ion (H₂PO₄ ⁻), hydrogen phosphate ion (HPO₄ ²⁻), and combinations thereof, dissociated from the phosphate. The phosphate solution is added to the pasty mixture and the phosphate radical ions react with the Al³⁺ located about the surface of the lithium cobalt oxide particle 12. Thus, a uniform aluminum phosphate layer can be formed on the surface of the lithium cobalt oxide particle 12 in situ. In one embodiment, the phosphate solution can be added to the pasty mixture drop by drop, while stirring the mixture, to react the phosphate radical ions entirely and evenly with the Al³⁺ on the surface of the lithium cobalt oxide particle 12. Similar to the AlPO₄ solution, the amount of the phosphate solution can be determined from the amount of the aluminum phosphate layer to be formed on the surface of the lithium cobalt oxide particle 12.

In the step S14, the heat treating step can firmly combine the aluminum phosphate layer and the lithium cobalt oxide particle 12 at the interfaces therebetween, to form the lithium cobalt oxide composite material. Meanwhile, the residual solvent and other reacting products (e.g. ammonium nitrate (NH₄NO₃)) can be removed from the final product. In addition, interface diffusion between the aluminum phosphate layer and the lithium cobalt oxide particle 12 may be occurred during the heat treating. The cobalt atoms of the lithium cobalt oxide particle 12 may be diffused into the aluminum phosphate layer. The heat treating temperature can be in a range from about 400° C. to about 800° C. A heat treating time period can be in a range from about 0.5 hours to about 2 hours.

In the method, the lithium cobalt oxide particles 12 are previously introduced to the Al(NO₃)₃ solution, and then the phosphate is added to react with the Al³⁺ on the surfaces of the lithium cobalt oxide particles 12, to in situ form the AlPO₄ layer coated on the lithium cobalt oxide particles 12. The liquid phase of Al(NO₃)₃ solution is mixed with the solid phase of lithium cobalt oxide particles 12. Therefore, the Al³⁺ can be uniformly coated on the surfaces of the lithium cobalt oxide particles 12. Accordingly, the in situ formed aluminum phosphate layer can also be uniformly coated on the surfaces of the lithium cobalt oxide particles 12. Instead of forming the AlPO₄ particles and then absorbing the AlPO₄ particles by the surfaces of the lithium cobalt oxide particles 12, the uneven absorption between the solid phase of AlPO₄ and the solid phase of lithium cobalt oxide particles 12 is avoided. Therefore, an uneven coating of the lithium cobalt oxide particles 12 is avoided. The AlPO₄ layer 14 can coat a single lithium cobalt oxide particle 12, and each of the lithium cobalt oxide particles 12 that is subsequently introduced can have the AlPO₄ layer 14 coated on the surface thereof. The method is suitable for industrialization. In addition, the in situ formed AlPO₄ layer 14 is an integrated and continuous material layer having uniform thickness, not just a plurality of AlPO₄ particles joined together. In use of lithium ion battery, the AlPO₄ layer 14 can prevent an electron migration between the electrolyte and the lithium cobalt oxide particles 12 and allow the lithium ions to pass therethrough. Thus, the side reaction during the charge and discharge of the lithium ion battery between the electrolyte and the lithium cobalt oxide particles 12 can be suppressed even at a relatively high voltage. Therefore, the electrode composite material has improved chemical and thermal stabilities, even at a relatively high or low charge/discharge voltage, or a high rate.

The step S2 can further include steps of:

S21, assembling an electrochemical cell adopting the lithium cobalt oxide composite 10 as a cathode active material; and

S22, charging the electrochemical cell to deintercalating lithium ions from the lithium cobalt oxide particle 12, thereby achieving the cobalt oxide 120.

The electrochemical cell can be a lithium ion battery or other electrochemical battery. One embodiment of the lithium ion battery includes a cathode, an anode, and a non-aqueous electrolyte disposed between the cathode and the anode. The cathode electrode includes a cathode current collector and a cathode material layer disposed on a surface of the cathode current collector. The anode electrode includes an anode current collector and an anode material layer disposed on a surface of the anode current collector.

The cathode material layer can include the lithium cobalt oxide composite, a conductive agent, and a binder mixed with each other. A ratio between the lithium cobalt oxide composite, the conductive agent, and the binder can be a common ratio used in a lithium ion battery. In one embodiment, the mass percentage of the lithium cobalt oxide in the total mass of the cathode material layer can be in a range from about 70% to about 98%. The anode material layer can include an anode active material, a conductive agent, and a binder. The anode active material can be lithium metal, lithium titanate, graphite, acetylene black, organic cracking carbon, mesocarbon microbeads (MCMB), or any combination thereof. The conductive agent can be at least one of acetylene black, carbon fibers, carbon nanotubes, and graphite. The binder can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and styrene-butadiene rubber (SBR). The non-aqueous electrolyte can be a solid film or a solution including a lithium salt dissolved in an organic solvent. The lithium salt may be at least one of LiPF₆, LiBOB, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, LiCF₃SO₃, Li(CF₃SO₂)₂N, LiC₄F₉SO₃, LiSbF₆, LiAlO₄, LiAlCl₄, LiCl, and LiI. The organic solvent can be a cyclic carbonate and linear carbonate, and can be at least one of ethylene carbonate (EC), propylene carbonate (PC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC). The lithium ion battery can further include a porous membrane or a separator located between the cathode electrode and the anode electrode. The material of the separator can be polypropylene (PP) or polyethylene (PE). The lithium ion battery can further include an exterior encapsulating structure such as a hard battery case or a soft encapsulating bag. The lithium ion battery can further include a connecting component achieving an electrical connection between the current collector of the lithium ion battery and the external circuit.

In the step S22, the lithium cobalt oxide has an electrochemical reaction to deintercalate the lithium ions from the LixCo_(1-y)M_(y)O₂, and thus forming the Co_(1-y)M_(y)O₂. More specifically, the lithium ion battery can be galvanostatic charged, constant-voltage charged, or the combination thereof. In one embodiment, all the lithium ions can be deintercalated from the lithium cobalt oxide. Vs can be in a range of 4.5V<Vs≦5V, wherein Vs is a cut-off voltage of the galvanostatic charge or the voltage of the constant-voltage charge. The lithium ion battery can be galvanostatic charged to 5V, or constant-voltage charged at 5V for a period of time. The time of the constant-voltage charge is related to the amount of the lithium cobalt oxide. In one embodiment, the time is about 20 minutes to about 20 hours. During the charging process, the lithium ions are deintercalated from the lithium cobalt oxide particle, go through the AlPO₄ layer 14, and intercalate into the anode electrode. Referring to FIG. 3, for the LiCoO₂ as an example, when the lithium ions are deintercalated from the LiCoO₂, CoO₂ having layered structure is left. The AlPO₄ protects the CoO₂ from being reduced to a lower valence state of the cobalt. The interfacial diffusion can occur at the interface between the AlPO₄ layer 14 and the lithium cobalt oxide particle 12, and cobalt atoms of the lithium cobalt oxide particle 12 may diffuse into the AlPO₄ layer 14, to form a Co—Al—O co-oxide. The Co—Al—O co-oxide can prevent or suppress the reduction of the cobalt in +4 of the CoO₂, to make the CoO₂ be existed stably.

It is to be understood that, the electrochemical lithium-deintercalating step of S2 may not be processed in the electrochemical cell, and can be processed at other devices as long as substantially all the lithium ions are deintercalated. For example, the lithium cobalt oxide composite 10 can be arranged in an electrolytic cell as a positive electrode to have the lithium-deintercalation.

A uniform thickness and continues of AlPO₄ layer is coated on the surface of the lithium cobalt oxide particle 12 to improve the electrochemical stability of the lithium cobalt oxide at a relatively high voltage to deeply deintercalate the lithium ions from the lithium cobalt oxide, thereby achieving the cobalt (IV) oxide. The cobalt (IV) oxide has a layered structure and can be used at least in a primary lithium battery.

EXAMPLES Example 1

In Example 1, the lithium cobalt oxide composite is an AlPO₄—LiCoO₂ composite material including LiCoO₂ particles and an AlPO₄ layer 14 coated on a surface of each of the LiCoO₂ particles. The mass percentage of the AlPO₄ layer in the cathode composite material is about 1%.

In the method of preparing the lithium cobalt oxide composite, Al(NO₃)₃ is dissolved in ethanol to form the Al(NO₃)₃ solution. 100 g of LiCoO₂ particles is introduced to the 30 mL and 0.16 mol/L of Al(NO₃)₃ solution, and stirred to form a pasty mixture. Water solution of (NH₄)₂HPO₄ is added to the pasty mixture drop by drop. Then, the mixture is uniformly stirred until the AlPO₄ is totally precipitated on the surfaces of the LiCoO₂ particles. The LiCoO₂ particles coated by the AlPO₄ layer are heated at about 400° C. Referring to FIG. 4 of the SEM photo, in the product, the AlPO₄ layer is uniformly coated on the surface of the LiCoO₂ particle. Referring to FIG. 5 of the TEM photo, the AlPO₄ layer formed is a material layer having uniform thickness coated on the surface of the LiCoO₂ particle.

Example 2

In Example 2, the lithium cobalt oxide composite is an AlPO₄—LiCoO₂ composite material prepared in the same method as in Example 1, except for heating the LiCoO₂ particles coated by the AlPO₄ layer at about 500° C.

Example 3

In Example 3, the lithium cobalt oxide composite is an AlPO₄—LiCoO₂ composite material prepared in the same method as in Example 1, except for heating the LiCoO₂ particles coated by the AlPO₄ layer at about 600° C.

Example 4

In Example 4, the lithium cobalt oxide composite is an AlPO₄—LiCoO₂ composite material prepared in the same method as in Example 1, except that the mass percentage of the AlPO₄ layer in the cathode composite material is about 1.5% and the LiCoO₂ particles coated by the AlPO₄ layer are heated at about 600° C.

Electrochemical Experiment 1

A half cell is assembled by using the lithium cobalt oxide composite formed in Example 1. The lithium cobalt oxide composite formed in Example 1, acetylene carbon black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder are mixed in N-methyl pyrrolidone (NMP) in a mass ratio of about 90:5:5, to form a slurry. The slurry is applied to the surface of an aluminum foil and dried at about 100° C. to remove NMP, and achieve the cathode electrode. The anode electrode is lithium metal. 1 mol/L LiPF₆/EC+DEC (1:1) is used as an electrolyte solution. The separator is a porous polypropylene film. The assembled half cell is cycled at a current of about 0.5 (C-rate) between a discharge cut-off voltage of 2.7V and a charge cut-off voltage of 4.5V at room temperature.

Electrochemical Experiments 2-4

Three half cells are assembled and cycled according to the same conditions as in the Electrochemical Experiment 1, except that the cathode composite material formed in Example 1 is respectively replaced by the cathode composite materials in Examples 2-4.

Referring to FIG. 6, the half cells of Examples 1-4 all use uniformly coated LiCoO₂ particles with AlPO₄ layer as the cathode active material, and have relatively high capacity in and capacity retention (i.e., capacity maintenance rate). After 50 cycles, the capacity retentions of the half cells are all above 90%, and the capacities are in a range from about 160 mAh/g to about 175 mAh/g. The half cell having the higher heat treating temperature has a higher capacity. However, the change in mass percentage of the AlPO₄ layer in the cathode composite material from 1% to 1.5% does not have much effect to the capacity of the half cell. The AlPO₄ layer modifies the surface structure of the LiCoO₂, provides a lithium intercalation/deintercalation platform, and works as a barrier layer to suppress the chemical reaction between Co⁴⁺ and the electrolyte. Thus, the AlPO₄ layer improves the stability of the LiCoO₂, and the cycling performance of the lithium ion battery is improved.

Referring to FIG. 7, the half cells are charged to 5V, and then the half cells are disassembled, and the cathode electrode is separated from the half cell to be examined by XRD analysis. As shown in the XRD pattern, the diffraction peaks of CoO₂ are labeled to prove that the layered type CoO₂ is achieved. Further, in FIG. 7, the first unlabeled peak from the left side of the pattern may be the peak of the residual Li_(x)CoO₂, and the relatively wide unlabeled peak at 20 degree to 30 degree may be the amorphous AlPO₄. The CoO₂ has a layered structure after the lithium ions are deintercalated therefrom, thus, the CoO₂ can be used as a cathode active material in a lithium ion battery to have the lithium ions intercalated therein.

Comparative Example

In Comparative Example 1, a comparative lithium cobalt oxide composite is prepared. A water solution of (NH₄)₂HPO₄ and a water solution of Al(NO₃)₃ are mixed together and stirred to form a mixture including a plurality of AlPO₄ particles dispersed in water. LiCoO₂ particles are added to the mixture, and the AlPO₄ particles are absorbed to the surfaces of the LiCoO₂ particles. The LiCoO₂ particles having the AlPO₄ particles absorbed thereon are filtered and heated at about 600° C., to form the comparative lithium cobalt oxide composite.

In the method, the pre-prepared AlPO₄ particles are added in water, and then the LiCoO₂ particles are mixed with the AlPO₄ in the water. The AlPO₄ particles adhere to the surface of the LiCoO₂ particles. The water is then evaporated and the LiCoO₂ particles with the AlPO₄ particles absorbed thereon are heated to form the comparative lithium cobalt oxide composite. However, AlPO₄ is insoluble in water. Therefore, in the above method, AlPO₄ particles may not disperse sufficiently in the water and could aggregate together. Further, when a large amount of the LiCoO₂ particles is added in the water, the LiCoO₂ particles added earlier in time will absorb the majority of the AlPO₄ particles in the water, and the subsequently added LiCoO₂ particles may not have enough AlPO₄ particles to absorb. The uneven absorption occurs most often when adding a relatively large amount of LiCoO₂ particles, which is a problem for industrialization of this composite material. Furthermore, referring to FIG. 8, even though it may appear that the LiCoO₂ is coated well, a microscopic view of the AlPO₄ may show that the small particles 22 coated on the surface of the LiCoO₂ large particle 24 do not form a uniform AlPO₄ substance layer. Accordingly, the LiCoO₂ cannot be stable at relatively high charging voltage, and the lithium ions cannot be thoroughly deintercalated therefrom. Referring to FIG. 9 and FIG. 10, it can be seen from the SEM photo that, the AlPO₄ is in a particle shape aggregated on the surface of the LiCoO₂ particle. The AlPO₄ particles are aggregated together, and the coating is not uniform.

Comparative Experiment 1

A Half cell is assembled and cycled according to the same conditions as in Electrochemical Experiment 1, except that the lithium cobalt oxide composite formed in Example 1 is replaced by uncoated LiCoO₂ particles.

Comparative Experiment 2

A half cell is assembled and cycled according to the same conditions as in Electrochemical Experiment 1, except that the lithium cobalt oxide composite formed in Example 1 is replaced by the comparative lithium cobalt oxide composite in Comparative Example.

Referring to FIG. 11, even charged to a relatively low voltage of 4.5V, the half cells of the comparative lithium cobalt oxide composite in Comparative Example 1 and the uncoated LiCoO₂ particles both have a rapid drop of capacity. After 50 cycles, the capacity retentions of the two half cells are both smaller than 85%. This is mainly because the uneven coating or uncoated LiCoO₂ is relatively unstable at relative high charge cut-off voltage. At the high charge cut-off voltage, the LiCoO₂ may have a side reaction with the electrolyte, to decrease the capacity. Thus, the CoO₂ cannot be achieved by charging the half cells.

Depending on the embodiment, certain steps of methods described may be removed, others may be added, and the sequence of steps may be altered. It is also to be understood that the description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.

Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the present disclosure. Variations may be made to the embodiments without departing from the spirit of the present disclosure as claimed. Elements associated with any of the above embodiments are envisioned to be associated with any other embodiments. The above-described embodiments illustrate the scope of the present disclosure but do not restrict the scope of the present disclosure. 

What is claimed is:
 1. A method for making a composite of cobalt oxide comprising steps of: providing an aluminum nitrate solution; introducing a plurality of lithium cobalt oxide particles into the aluminum nitrate solution; mixing the plurality of lithium cobalt oxide particles with the aluminum nitrate solution to form a mixture; adding a phosphate solution into the mixture to react with the aluminum nitrate solution and form an aluminum phosphate layer on surfaces of the plurality of lithium cobalt oxide particles; heat treating the plurality of lithium cobalt oxide particles with the aluminum phosphate layer formed on the surfaces thereof to form a lithium cobalt oxide composite; and obtaining a cobalt oxide by electrochemical lithium deintercalation in the lithium cobalt oxide composite at a voltage of Vx, wherein 4.5V<Vx≦5V, a material of the cobalt oxide is represented by a chemical formula of Co_(1-y)M_(y)O₂, wherein 0≦y≦0.9, and M is selected from the group consisting of alkali metal elements, alkaline-earth metal elements, Group-13 elements, Group-14 elements, transition metal elements, and rare-earth elements.
 2. The method of claim 1, further comprising controlling a volumetric ratio of the aluminum nitrate solution and the plurality of lithium cobalt oxide particles to make a pasty mixture.
 3. The method of claim 1, wherein the aluminum nitrate solution comprises a liquid phase solvent and an aluminum nitrate dissolved in the liquid phase solvent.
 4. The method of claim 3, wherein the liquid phase solvent is ethanol.
 5. The method of claim 1, wherein the phosphate solution comprises a liquid phase solvent and a phosphate dissolved in the liquid phase solvent, the phosphate is selected from the group consisting of monoammonium phosphate, diammonium phosphate, ammonium phosphate, phosphoric acid, and combinations thereof.
 6. The method of claim 1, wherein a temperature of the heat treating is in a range from about 400° C. to about 800° C.
 7. The method of claim 2, wherein the volumetric ratio of the aluminum nitrate solution and the plurality of lithium cobalt oxide particles is in a range from about 1:10 to about 1:40.
 8. The method of claim 1, wherein an amount of the aluminum nitrate solution is about 30 mL, and an amount of the plurality of lithium cobalt oxide particles is about 100 g.
 9. The method of claim 1, wherein the electrochemical lithium deintercalation comprises: assembling an electrochemical cell adopting the lithium cobalt oxide composite as a cathode active material; and charging the electrochemical cell.
 10. The method of claim 9, wherein the electrochemical cell is charged to about 5V.
 11. The method of claim 1, wherein a mass percentage of the aluminum phosphate layer is in a range from about 0.1% to about 3%.
 12. The method of claim 1, wherein a mass percentage of the aluminum phosphate layer is in a range from about 1% to about 1.5%.
 13. The method of claim 1, wherein a thickness of the aluminum phosphate layer is in a range from about 5 nanometers to about 20 nanometers.
 14. The method of claim 1, wherein the aluminum phosphate layer is continuous and has a uniform thickness.
 15. A method for making a composite of cobalt oxide comprising steps of: providing an aluminum nitrate solution; introducing a plurality of lithium cobalt oxide particles into the aluminum nitrate solution; mixing the plurality of lithium cobalt oxide particles with the aluminum nitrate solution to form a mixture; adding a phosphate solution into the mixture to react with the aluminum nitrate solution and form an aluminum phosphate layer on surfaces of the plurality of lithium cobalt oxide particles; heat treating the plurality of lithium cobalt oxide particles with the aluminum phosphate layer formed on the surfaces thereof to form a lithium cobalt oxide composite; and obtaining cobalt (IV) oxide with all lithium ions being deintercalated from lithium cobalt oxide by electrochemical lithium deintercalation in the lithium cobalt oxide composite at a voltage of Vx, wherein 4.5V<Vx≦5V.
 16. A method for making a composite of cobalt oxide comprising steps of: providing an aluminum nitrate solution; introducing a plurality of lithium cobalt oxide particles into the aluminum nitrate solution; mixing the plurality of lithium cobalt oxide particles with the aluminum nitrate solution to form a mixture; adding a phosphate solution into the mixture to react with the aluminum nitrate solution and form an aluminum phosphate layer on surfaces of the plurality of lithium cobalt oxide particles; heat treating the plurality of lithium cobalt oxide particles with the aluminum phosphate layer formed on the surfaces thereof to form a lithium cobalt oxide composite; and obtaining CoO₂ by electrochemical lithium deintercalation in the lithium cobalt oxide composite at a voltage of Vx, wherein 4.5V<Vx≦5V. 